An example of the angular rate sensor of the kind described is disclosed in U.S. Pat. No. 5,349,855 entitled "COMB DRIVE MICROMECHANICAL TUNING FORK GYRO". Another example is disclosed in Japanese Laid-Open Patent Application No. 43,166/95 entitled "ANGULAR RATE SENSOR". Each of these examples is a microgyro obtained by a micromachining of silicon using a semiconductor processing process.
An angular rate sensor of the kind described includes an oscillator in the form of a flat plate disposed in an x-y plane and having two sets of comb tines, each set disposed on a lateral end or a side extending parallel to the y-axis, with pairs of support limbs extending in the y-direction from the respective longitudinal ends or sides extending parallel to the x-axis, the limbs supporting the plate oscillator in a suspended manner to permit its oscillation in x- and z-direction. A first and a second set of stationary comb tines are disposed on the outside of the lateral ends of the oscillator in an interdigitated manner with the first and the second set of comb tines on the oscillator while avoiding a contact therebetween and maintaining a microgap therebetween. An a.c. voltage of a frequency f is applied across the oscillator and the first and the second set of stationary comb tines so that the oscillator is alternately attracted by the electrostatic attraction from the first and the second set of stationary comb tines for oscillation in the x-direction at the frequency f.
When an angular rate of rotation about the y-axis is applied to the oscillator while the latter is oscillating in the x-direction, Coriolis force is applied to the oscillator, which then undergoes an elliptical motion comprising the oscillation in the x-direction on which an oscillation in z-direction is superimposed. Thus an oscillation in the z-direction appears in the oscillator. An electrode is disposed in opposing relationship with the oscillator with a microgap therebetween and has a capacitance which varies in accordance with the oscillation in the z-direction. The variation has an amplitude which is approximately inversely proportional to the amplitude of the oscillation of the oscillator in the z-direction. By converting the capacitance thus determined into a corresponding electrical signal level or analog voltage, which represents a capacitance detection signal, there is obtained a voltage having an amplitude which is inversely proportional to the amplitude of the oscillator in the z-direction. Since this amplitude corresponds to the value of the angular rate, a synchronized detection of the capacitance detection signal in synchronism with an exciting signal applied to the oscillator allows a d.c. voltage having a level which corresponds to the value of the angular rate to be obtained.
Representing the mass of the oscillator by m, the amplitude of the oscillation by a, the period by .omega. and the angular rate by .OMEGA., a maximum value of the rate of oscillation is given by a .omega.. Accordingly, the Coriolis force Fc has a maximum value Fcmax, which is defined as follows: EQU Fcmax=2m .OMEGA. a .omega.
The Coriolis force Fc has a magnitude which is proportional to both a and .omega.. However, for a microgyro, a range over which .omega. is varied is limited by the structure of the microgyro. Accordingly, a Coriolis force Fc having an increased magnitude is developed by increasing the amplitude a. In order to generate the Coriolis force Fc most efficiently, an electrical drive circuit is designed to drive the oscillator for oscillation at its resonant frequency. Due to manufacturing errors, however, the resonant frequency varies from oscillator to oscillator, requiring a tuning of the drive circuit for each angular rate sensor. However, the smaller the size of the sensor, the greater the manufacturing errors or the variation, whereby a difficulty is involved in the tuning.
In addition, the oscillation of the oscillator tends to be unstable due to the oscillations of the resonant frequency in the x-and y-directions and due to a change of the resonant frequency with the passage of time. It is difficult to accommodate the tuning to the change of the resonant frequency with the passage of time from power ON or after a long usage of the sensor.
The angular rate sensor disclosed in the Japanese Laid-Open Patent Application No. 43,166/95 has a gap at an end of a support member for supporting an oscillator. An electrode is disposed at the gap. An electric voltage is applied across the support member and the electrode. The gap is adjusted for maximizing an amplitude of the oscillator at a design frequency by adjusting a level of the electric voltage. However, a range of the adjustment of the level is relatively narrow. Thus the adjustment is insufficient to the manufacturing errors or the change of the resonant frequency with the passage of time. Since an actual resonant frequency of the oscillator is unknown, a frequency of an oscillation drive signal which is generated in an electric drive circuit for energizing the oscillator can not be adjusted to the actual resonant frequency.